The present invention relates to a method of manufacturing a III-V compound semiconductor which includes nitrogen and at least another Group V element, by irradiating a substrate with molecular beams of source materials in a growth chamber being so evacuated that a mean free path of the molecular beams is larger than a distance between the substrate and a molecular beam source, as in a molecular beam epitaxy (MBE) method, a gas source MBE (GSMBE) method, a chemical beam epitaxy (CBE) method, a metal organic molecular epitaxy (MOMBE) method, and so on.
Recently, III-V compound semiconductors which each include nitrogen and at least another Group V element have been drawing attention as optoelectronic materials, because the lattice constants and energy bandgaps of the III-V compound semiconductors can been controlled in wide ranges by changing their mixed crystal compositions. Among others, GaInNAs has a bandgap desirable for an active layer of a laser used for optical fiber communication which emits light in a band including a wavelength of 1.3 xcexcm or 1.5 xcexcm and its composition enables lattice matching with GaAs, and can attain a large band offset of its conduction band through combination with AlGaAs or InGaP. Therefore GaInNAs is expected to be a material capable of implementing a semiconductor laser for optical fiber communication which has an improved temperature characteristic that necessary current for light emission does not increase very much even if temperature increases (see, for example, OYO BUTSURI, vol. 65, 1996, p. 148).
For crystal growth using the molecular beam epitaxy (MBE) method, the gas source MBE (GSMBE) method, the chemical beam epitaxy (CBE) method, the metal organic molecular epitaxy (MOMBE) method and the like (hereinafter, these methods are generically referred to as the MBE method) which are used for forming a III-V compound semiconductor, including nitrogen and another Group V element, such as GaInNAs, active nitrogen (nitrogen radical) which is generated from plasma of nitrogen molecules (N2) or ammonia (NH3) has been used in view of the incorporation efficiency of nitrogen into a crystal to be grown (see Japanese Patent Laying-Open No. 6-334168).
By using to the MBE method which uses nitrogen radical as a nitrogen source, however, the crystal growth of a III-V compound semiconductor which includes nitrogen and another Group V element has a problem that crystallinity of the semiconductor decreases with increase of the nitrogen concentration. In a semiconductor laser in which Ga0.75In0.25N0.005As0.995 is used for an active layer of a single quantum well (SQW), for example, laser oscillation at a wavelength of 1.113 xcexcm at 77K has been reported (see Electronics Letters, vol. 32, 1996, p. 1586). However, laser oscillation in the band including a wavelength of 1.3 xcexcm or 1.55 xcexcm, which is used for optical fiber communication, has not been achieved.
In order to obtain an energy bandgap which corresponds to the oscillation wavelength of at least 1.3 xcexcm in the GaInNAs type mixed crystal capable of lattice matching with GaAs, the nitrogen content in V group elements has to be at least about 2.5 at. % larger than conventional 0.5 at. %. It has been found out however that the crystallinity decreases with increase of the nitrogen content in the semiconductors grown by the conventional MBE method using a nitrogen radical.
This could be because when a nitrogen radical is used as the nitrogen source, crystal defects are induced by high reactivity or high energy of the nitrogen radical. Due to the high reactivity of nitrogen radical, nitrogen easily bonds with the III group element and is incorporated at high efficiency into the growing crystal. At the same time, nitrogen easily bonds with the V group element other than nitrogen and forms nitrogen compound molecules. Since the nitrogen compound molecules are also incorporated into the crystal, crystal defects such as a Group V element at an antisite (Group V element at a lattice site which should be occupied by the Group III element) and interstitial atoms may be induced. At this time, even if Nxe2x80x94N bonds are produced, N2 molecules have a high vapor pressure, and they are less likely to be incorporated into the crystal. Thus, the crystallinity is lowered when the Group V element other than nitrogen bonds with nitrogen.
It is further considered that energy emitted from nitrogen radical dissociates the bond between the Group III element and the Group V element other than nitrogen (Gaxe2x80x94As bond, for example) in the vicinity of growth surface, desorbing the Group V element, which has a relatively high vapor pressure, and thus creating vacancies. Since the bond between the Group III element and nitrogen is strong and stable, the crystallinity is lowered especially in a III-V compound semiconductor which includes another Group V group element having a weaker bonding strength with the III group element as compared with nitrogen.
Therefore, when a crystal of the III-V compound semiconductor which includes nitrogen and another Group V element is to be grown by the MBE method using nitrogen radical, the increased amount of nitrogen radical supply to increase the nitrogen content in the crystal makes it difficult to obtain a high quality crystal.
In view of the above described problems with the conventional art, an object of the present invention is to provide a method of manufacturing a compound semiconductor capable of achieving superior crystallinity even in a III-V compound semiconductor which includes nitrogen and at least another Group V element.
A method of manufacturing a compound semiconductor according to one aspect of the present invention is characterized in that a nitrogen compound is used as a nitrogen source, molecules of the nitrogen compound decompose after they reach a surface of a substrate and only nitrogen atoms are incorporated into a III-V compound semiconductor crystal including nitrogen and at least another Group V element when the crystal is grown by irradiating the substrate with molecular beams of source materials in a growth chamber being so evacuated that a mean free path of the molecular beams is larger than a distance between the substrate and molecular beam sources.
In other words, nitrogen atoms are incorporated into the crystal through dissociation and adsorption of the nitrogen compound at the growth surface. Therefore, the reaction of the Group V element other than nitrogen with nitrogen can be suppressed, when the nitrogen compound decomposes, by using the stable nitrogen compound rather than nitrogen radical as the nitrogen sources. Since energy emitted by the reaction in which nitrogen is incorporated into the crystal through the decomposition of the nitrogen compound at the growth surface can be made smaller than energy emitted from nitrogen radical, the dissociation of the bond between the Group III element and the Group V element other than nitrogen in the vicinity of the growth surface can be suppressed, thereby enabling stable crystal growth.
Nitrogen hydride can preferably be used as the nitrogen compound. Since the dissociation energy of nitrogen hydride is smaller compared to an N2 molecule, the dissociation of the hydride and the adsorption of nitrogen at the growth surface can occur at a relatively low temperature. Therefore, the composition ratio of nitrogen and another Group V element in the compound semiconductor to be grown can be controlled easily. Since H2 molecules which are produced by the dissociation of the nitrogen hydride are easily desorbed from the growth surface, undesirable impurities are not incorporated into the crystal. Further, when impurities such as carbon (C) exist at the growth surface, a cleaning effect in which the impurities form hydrides and are adsorbed from the growth surface can be expected.
NH3 can preferably be used as the nitrogen hydride. NH3 is the most stable of nitrogen hydrides, and it can minimize generation of crystal defects, which are caused by the interaction of nitrogen and another Group V element, when nitrogen atoms are incorporated into the crystal through dissociation and adsorption.
When a nitrogen hydride is used as the nitrogen compound, the substrate temperature is preferably maintained in a range of 500 to 750xc2x0 C. during crystal growth. When NH3 is used as the nitrogen compound, NH3 is preferably heated to a temperature in a range of 350 to 500xc2x0 C. before it is directed to the substrate.
It is noted that hydrazine (N2H4) is also preferably used similarly to NH3 as the nitrogen hydride.
An alkylated nitrogen compound is also preferably used as the nitrogen compound. In general, an alkylated nitrogen compound has smaller bond dissociation energy than nitrogen hydride and this results in easier dissociation of the nitrogen compound at the growth surface and increase of the nitrogen incorporation efficiency into the crystal. Since the alkyl group produced by the decomposition of the alkylated nitrogen compound is hydrocarbon having a high vapor pressure, it is easily desorbed from the growth surface and not incorporated into the crystal. Thus, a high purity crystal can be obtained.
It is noted that an alkylated hydrazine compound can also preferably be used as the alkylated nitrogen compound.
Further, alkylamine can also preferably be used as the alkylated nitrogen compound. When the stable alkylamine is used, generation of crystal defects caused by the interaction of nitrogen and another Group V element can be minimized while nitrogen atoms are incorporated into the crystal through dissociation and adsorption at the growth surface. When the alkylamine compound is used as the nitrogen compound, the substrate temperature is preferably maintained in a range of 400 to 750xc2x0 C. during crystal growth.
The substrate is preferably a compound semiconductor which has a zinc blende structure. The substrate surface preferably has a prescribed off-angle from a {100} plane to a {111}A plane. At the surface of such a compound semiconductor substrate, decomposition of the nitrogen compound is promoted, and the high incorporation efficiency of nitrogen into the crystal can be achieved even when the nitrogen compound which is lower in reactivity than nitrogen radical is used. Especially, the off-angle of the substrate surface is preferably in a range of 5 to 15xc2x0.
When the Group V element is supplied on the substrate, the process of directing nitrogen compound molecules to the growth surface and the process of directing another Group V element to the growth surface are preferably performed alternately without being overlapped. By separately supplying the nitrogen material and another Group V element on the substrate, the interaction of nitrogen and another Group V element can be reduced. Therefore, the incorporation efficiency of nitrogen into the crystal can be improved, and further the reaction of nitrogen and another Group V element can be fully suppressed, which can not fully be achieved only by using one of nitrogen materials other than nitrogen radicals.
A nitrogen compound which has a Gibbs energy of formation of less than 100 kJ/mol is more preferably used as the nitrogen source. By using the nitrogen compound with the Gibbs energy of formation of less than 100 kJ/mol, the reaction of nitrogen and the Group V element other than nitrogen can be suppressed more effectively during the decomposition of the nitrogen compound. At the growth surface, the dissociation of the bond between the Group III element and the Group V element other than nitrogen, possibly caused by the reaction in which nitrogen is incorporated into the crystal through decomposition of the nitrogen compound, can be suppressed more effectively. Thus, a high quality crystal can be obtained.